1. Field of the Invention
This invention concerns an aperture pattern-printing plate used for making a shadow mask, and a method of manufacturing the same.
2. Description of the Related Art
Shadow masks commonly used for color cathode ray tubes have a large number of apertures. These shadow masks are used to allow three electron beams corresponding to red, green, and blue emitted from the electron gun to impinge on each corresponding phosphor through the apertures. They are usually manufactured by a photo-etching process, for example as described below.
First, a shadow mask substrate consisting of a continuous strip of metal sheet is degreased and washed, and a photoresist layer of a given thickness is formed on both the principal surfaces of the mask.
Second, a pair of printing plates, each corresponding to the aperture pattern of a shadow mask, are laid over the respective principal surfaces of the shadow mask substrate in tight contact therewith. The printing plates are opaque at portions corresponding to the apertures of the shadow mask, and are transparent at the other portions. The two printing plates are similar to each other in the arrangement of the opaque portions, but the opaque portions of one printing plate is larger than the opaque portions of the other printing plate in an ordinary case. The photoresist layers are exposed to the ultraviolet light passing through the respective printin plates. The unexposed parts of the photoresist layers corresponding to the apertures of the shadow mask are dissolved and are removed by a warm water spray. The mask substrate is dried and baked so as to leave a residual photoresist layer resistant to etching at points other than the apertures. An etchant is sprayed over the surfaces of the mask substrate to perforate the apertures. Then, the resultant structure is washed, and the photoresist layer is removed therefrom. After the removal of the photoresist layer, the structure is washed again and dried, to obtain a shadow mask.
FIG. 1 is a sectional view of an example of a printing plate used in the exposure step mentioned above. As is shown in FIG. 1, a flat glass substrate 1 is overlaid with an underlying layer 2. On this underlying layer 2, a flat emulsion layer 3 is formed by coating the layer 2 with a suspension containing silver halide dispersed in gelatin. The emulsion layer 3 is made up of opaque portions 4 and transparent portions 5. The opaque portions 4 correspond in location to the respective apertures of a shadow mask, so that the opaque portions 4 and the transparent portions 5 jointly constitute a shadow mask pattern. The underlying layer 2 is formed for the purpose of improving the coating and adhering properties of the emulsion layer 3.
The shadow mask pattern, which is constituted by the opaque and transparent portions 4 and 5 of the emulsion layer 3, is formed as follows. First, an original pattern plate is prepared by use of a pattern generator which is generally referred to as a photo plotter. An unexposed photosensitive glass plate is stacked upon, and brough into tight contact with the original pattern plate. A master pattern, whose pattern is reverse to that of the original pattern plate, is prepared by exposing the photosensitive glass plate to the light passing through the original pattern plate. It should be noted that the master pattern may have a pattern defect, such as a pattern defect originating from the unexposed photosensitive glass plate or a pattern defect arising from foreign matter entering the region between the original pattern plate and the photosensitive glass. If such a pattern defect is found in the master pattern, it is corrected or removed manually. In a darkroom, thereafter, the master pattern is stacked upon, and brought into vacuum contact with a photosensitive plate having an unexposed emulsion layer 3. By use of a 200-watt mercury lamp, the photosensitive plate is exposed to the light passing through the master pattern. The photosensitive plate, thus exposed, is then subjected to ordinary development processing, including developing, first washing, fixing, second washing, drying, etc. As a result, a pattern printing plate having a pattern equivalent to the original pattern, is obtained.
The pattern printing plate obtained in the abovementioned manner is attached to an exposure device, which can hold the pattern printing plate vertically. As such an exposure device, the exposure device disclosed in e.g. Published Examined Japanese Patent Application (PEJPA) No. 56-13298 is employed. The emulsion layer of the pattern printing plate attached to the exposure device is brought into vacuum contact with the photoresist film formed on a shadow mask substrate, and the photoresist film on the shadow mask substrate is exposed to the light emitted from the exposure device.
In the process of successively manufacturing shadow masks, a continuous strip of shadow mask material coated with a photoresist film is fed, brought into tight contact with a pattern printing plate, exposed to light, and wound up. In the manufacturing process wherein those operations are repeatedly performed, it may happen that the support frame used for supporting a pattern printing plate will have a mechanically-assembling error (including a clearance). It may also happen that the support frame and the pattern printing plate will ununiformly expand, due to the heat generated by the exposure device. In such cases, the pattern printing plate and the shadow mask material are inevitably shifted from each other though slightly, so that the emulsion layer of the pattern printing plate and the photosensitive film of the shadow mask substrate are abraded against each other. Even if the above operations are performed in a dust-free room, dust particles or other undesirable particles are inevitably produced in the meantime. For example, the metallic burrs left on the shadow mask material, the residual pieces of resist, the paint pieces and metallic powder dropping off the exposure device, and the dust particles flying up as the operator walks on the floor constitute the undesirable particles mentioned above. It is inevitable that such undesirable particles will stick to the photoresist film or the emulsion layer. It is also inevitable that such particles will enter the region between the photoresist film and the emulsion layer when these two are brought into tight contact with each other. It should be also noted that the photoresist film of the shadow mask substrate is baked appropriately at 200.degree. C. and has a pencil hardness of about 5H to 6H, whereas the emulsion layer of the pattern printing plate is not baked and has a low pencil hardness of about B to lH. Therefore, if the emulsion layer of the pattern printing plate is abraded against the photoresist film or has dust particles sticking thereto, it is very likely that the emulsion layer will be damaged, resulting in a pattern defect. If such a pattern defect is generated, all shadow masks produced thereafter will have defective apertures.
To shove this problem, it is thought to cover the emulsion film with a surface film formed of acrylic resin or amino alkyd resin. However, in order to provide the surface film with a hardness equivalent to or higher than that of the photoresist film, the thickness of the surface film has to be at least 10 .mu.m.
FIG. 2 is a cross sectional view illustrating the state where a shadow mask material and a pattern printing plate provided with a surface film have been brought into tight contact with each other, for light exposure. As is shown in FIG. 2, the pattern printing plate is made up of: a glass substrate 1; an underlying layer 2 formed on the glass substrate 1; an emulsion layer 3 which is formed on the underlying layer 2 and which has opaque and transparent portions 4 and 5 jointly constituting a shadow mask pattern; and a surface film 9 made of e.g acrylic resin and formed on the emulsion layer 3. The shadow mask material is made up of a shadow mask substrate 8, and a photoresist film 7 formed on the shadow mask substrate 8. The shadow mask material is stacked on the pattern printing plate, with its photoresist film 7 in tight contact with the surface film 9 of the pattern printing plate. The photoresist film 7 has a pattern 10 formed by exposing the film 7 to the light passing through the shadow mask substrate 8. Normally, the emulsion layer 3 has a thickness of about 6 .mu.m, and the photosensitive film 7 has a thickness in the range of approximately 5 to 10 .mu.m. Since the surface film 9 is sandwiched between the photosensitive film 7 and the emulsion layer 7, as is depicted in FIG. 2, the photoresist film 7 and the emulsion layer 3 are located away from each other by the distance corresponding to the thickness of the surface film 9. If the photosensitive film 7 and the emulsion layer 3 are away from each other more than a certain distance, a latent pattern image to be printed o the photosensitive film 7 in the exposure step is adversely affected by light diffusion. In particular, the dimensions of the latent pattern image 10 may change in accordance with a variation in the distribution of the light emitted from the light source or a variation in the exposure time. In light of this point, the surface film 9 should be as thin as possible.
In recent years, larger-sized color cathode ray tubes have come into general use. In accordance with this trend, pattern printing plates and their pattern areas have been increased. This being so, it has become difficult to uniformly form the surface film 9 having a given thickness. In other words, it is likely that the surface film 9 will have uneven thickness. If the surface film 9 has such uneven thickness, a shadow mask will have apertures of different dimensions, thus degrading the quality of a color cathode ray tube.
Even if the surface film 9 is hard enough to resist scratches, this is still insufficient. That is, the surface film 9 has to be prevented from being electrically charged and from being viscous. If the surface film 9 is electrically charged or viscous, foreign matter is liable to stick to the surface film 9. If the foreign matter sticking to the surface film 9 is so large as to adversely affect the dimensions of the apertures of the shadow mask, a defective pattern is produced in the exposure step. In addition, if the foreign matter sticking to the surface film 9 has a size exceeding the total thickness of the emulsion layer, photosensitive film and surface film combined, the pattern will be inevitably scratched. To solve this problem, it may be thought to form a charge-preventing film on the emulsion layer, so as to prevent foreign matter from electrostatically sticking to the emulsion layer. In general, however, such a charge-preventing film is not very hard, and its scratch resistance is lower than that of the photosensitive film of the shadow mask material. With such a charge-preventing layer, therefore, it is difficult to effectively prevent scratches from being left on the pattern.
Additionally, if the surface film 9 of the pattern printing plate is viscous or low in hardness, air cannot be easily removed from the interface between the pattern printing plate and the shadow mask material when these two are brought into vacuum contact with each other. Therefore, this vacuum contact step requires much time and cannot be performed at high efficiency.
As mentioned above, the conventional pattern printing plate is not strong against scratches, and produces a defective pattern if it is abraded or has foreign matter. If the pattern printing plate becomes defective during the process of successively manufacturing shadow masks, such a defective pattern printing plate has to be replaced with another, with the manufacturing operation being stopped. If such replacement is required a number of times, it is difficult to manufacture shadow masks at good yield.